As ever higher levels of performance are being sought in computing and communications devices, there is a need for increasingly sensitive methods and instruments for evaluating material and device characteristics such as, e.g., response times and pulse shapes. In this respect, however, the development of electronic sampling oscilloscopes of the currently available type is estimated as having reached a point at which significant further increases in response speed are unlikely. As a result, alternate methods are being sought especially for investigating signals having time-dependent characteristics which may range from a few hundred picoseconds down to the subpicosecond range.
One approach which represents an alternative to the use of an electronic sampling oscilloscope is based on the generation of electrical signals upon illumination of a device or photoconductive material by optical pulses as disclosed in U.S. Pat. No. 4,482,863, issued Nov. 13, 1984 to D. H. Auston et al. In this approach, optical pulses are used both to generate as well as to sample electrical events and, if the device or material being tested is photosensitive itself, optical pulses may be made incident directly on the device or material being tested. For further details in this respect see the papers by P. R. Smith et al., "Measurement of GaAs Field-effect Transistor Electronic Impulse Response by Picosecond Optical Electronics", Applied Physics Letters, Vol. 39 (1981), pp. 739-741 and by D. H. Auston et al., "Picosecond Optical Electronics for High-speed Instrumentation", Laser Focus Magazine, April 1982, pp. 89-93.
Another approach to the characterization of ultrafast signals is based on electrooptic sampling; this represents an electric field-sensitive technique in which ultrashort optical pulses serve as "sampling gates" for electrical transients as they simultaneously propagate through an electrooptic medium. In this respect, see U.S. Pat. No. 4,446,425, issued May 2, 1984 to J. A. Valdmanis et al., the paper by J. A. Valdmanis et al., "Picosecond Electro-optic Sampling System", Applied Physics Letters, Vo. 41 (1982), pp. 211-212, and the paper by J. A. Valdmanis et al., "Subpicosecond Electrooptic Sampling: Principles and Applications", IEEE Journal of Quantum Electronics, Vol. QE-22 (1986), pp. 69-78.
While methods disclosed in the cited references are based on sampling a repetitive train of pulses, methods are desired also for characterizing signals which are non-repetitive. Methods of such latter type, and devices for their implementation, not only are potentially useful for testing and measurement purposes but may be used also in the field of telecommunications.